Fig. 1.  Macrophiothrix koehleri, approximate arm length 30 cm (arms not shown in full).

Evolutionary change in egg size is thought to drive changes in a number of other life-history traits involving fertilization rate, larval form, growth rate, feeding mode, and planktonic duration. We are using comparative methods to test for these functional relationships in a large group of ophiuroids (brittlestars) in the genus Macrophiothrix.  Adults of twelve species at Lizard Island, Australia are similar in form and ecology (Fig. 1), but show greater than 60-fold variation in egg size, reflecting large differences in how species package resources into individual offspring.  We are taking advantage of this remarkably broad egg size range among close relatives to examine a key ecological and life-history transition--the evolution of nonfeeding larval development--and associated changes in larval form and function.

Fig. 2.  Phylogeny for 16 Ophiothrichid brittlestars (14 Macrophiothrix & 2 Ophiothrix) and an Ophionereid outgroup, based on two mitochondrial genes (16S rDNA and CO-I).   Relative egg sizes are shown to the right; brown bars across branches indicate the evolution of large egg size and a derived mode of larval development.  Numbers at nodes indicate bootstrap values and Bayesian posterior probabilities.  Hart and Podolsky (2005), Molecular Phylogenetics and Evolution.

A molecular phylogeny, developed in collaboration with Dr. Mike Hart (Simon Fraser U.), suggests that large egg size has evolved independently at least three and possibly four times in the genus (see Fig. 2). Analysis of development rate and size data using independent contrasts shows that evolutionary increase in egg size has been accompanied by earlier hatching, larger hatching size, faster skeletal growth, and earlier metamorphosis—benefits that could help to explain the repeated evolution of large size despite the fecundity cost.

Small and medium-size eggs (yellow and orange in Fig. 2) are associated with the typical and ancestral mode of larval development involving a morphologically complex feeding pluteus larva.  Large egg size (brown) is associated with at least three derived modes of development: facultatitive planktotrophy with pluteus morphology (M. rhabdota), nonfeeding development with pluteus morphology (M. caenosa), and nonfeeding development with highly simplified morphology (M. belli, M. nereidina).  This analysis provides the first evidence for a life-history transition, originally predicted by Wray (1996), involving stepwise changes in feeding necessity, feeding capacity, and feeding morphology that are resolvable in evolutionary time.

Having spent several field seasons accumulating comparative data on development, life-history mode, and reef characteristics at 14 collection sites, we are now using the phylogenetic hypothesis to test several theoretical predictions of covariation between egg size and (1) sperm characters (size and swimming speed), (2) adult size, (3) local distributions of Macrophiothrix species in relation to reef characteristics, and (4) the geographic distribution of species in the Indo-West Pacific.